It is important to remove insoluble components in water discharge from an industrial plant, purification treatment of wastewater, use of groundwater and the like. For the removal, a water treatment filter is generally used.
A mixture of microorganism and extra-cellular products, so-called biofouling, adheres to a water treatment filter in addition to that insoluble components are naturally accumulated due to the use of the filter. If these phenomena are neglected, plugging occurs. It is therefore necessary to periodically wash a water treatment filter. However, biofouling is not at all easy to be removed and it is difficult to completely remove biofouling, since biofouling is protection means of microorganism and is sometimes resistant even to certain kinds of chemical substances. Therefore, an oxidizing agent such as hypochlorite is used for washing a water treatment filter.
Hypochlorite is effective against a wide range of microorganism and capable of dissolving and removing biofouling. In addition, hypochlorite also shows an instantaneous effect, a bleaching action, a deodorization function and even more is highly water soluble, economical, relatively low toxic and noncombustible so that hypochlorite has been used widely in food industries and water treatment industrial fields.
Although hypochlorite is widely used for washing a water treatment filter as described above, hypochlorite causes a problem that a filter itself suffers damage and the life of the filter is shortened due to the high effect. Specifically, chlorine in hypochlorite exists in the form of Cl+, which is in extremely electron-poor state as compared with stable chloride ion (Cl−); and therefore, hypochlorite acts as a strong oxidizing agent and attacks the hydrophilic groups and the polymer backbone of a filter material, and sometimes eventually cuts the polymer backbone. It has been therefore studied to employ a material with high chemical resistance as a base material for a filter.
As a raw material excellent in chemical resistance, a fluororesin such as PTFE (polytetrafluoroethylene) can be exemplified. Actually, the fluororesin is utilized as a base material for a water treatment filter; however, there is a disadvantageous point that the fluororesin is inferior in hydrophilicity. Accordingly, technologies for improving the hydrophilicity of a filter made of a fluororesin have been developed.
For example, Patent Document 1 discloses a method of impregnating a porous PTFE membrane with a hydrophilic copolymer of a fluorine-containing vinyl monomer and a vinyl monomer having a hydrophilic group. Patent Documents 2 and 3 disclose methods of coating the surface of a porous PTFE membrane with a hydrophilic polymer such as a cross-linked polyvinyl alcohol.
Further, as a technology for improving the chemical resistance of a filter by surface treatment rather than by material innovation, Patent Document 4 discloses a filter provided with an antioxidant metal layer made of a metal for eliminating active species causing an oxidation action on the surface of a filter base material in order to improve the durability against the active species such as a hydroxy radical.
In addition, as a technology for suppressing the decrease of water permeability due to the repetitions between dry condition and wet condition, Patent Document 5 discloses a porous membrane coated with a cross-linking coating layer in which the cross-linking coating layer contains a blocked isocyanate or a urethane.